feat(admin): 平台商户成员管理改用抽屉，消除商户视角切换与全局状态污染

- 交互重构：在「平台商户」与「平台账号」点击成员管理时，不再跳转路由或改写 localStorage.merchant_id，改为滑出 MerchantMembersDrawer 抽屉
- 抽屉能力：支持查看指定商户成员列表、新增成员（新建账号或关联已有账号）、修改角色与启用状态、移除成员
- 顶栏清理：彻底移除顶栏右上角的「当前商户：xxx」标签与「退出商户视角」按钮，超管登录自动清理任何残留上下文
- 后端补齐：新增平台专属 /api/platform/merchants/:id/members 与 roles 接口，平台操作不再依赖 X-Merchant-ID
- 验证：tsc -b && vite build 成功；全量 go test ./... 包含路由与鉴权回归测试 100% 通过

frontend/src/pages/PlatformUsers.tsx

@@ -8,7 +8,7 @@ import type { MerchantMember, MerchantMemberGroup, PageResult, User } from '../t
 import { formatDateTime } from '../utils/time'
 import { useAuth } from '../store/authContext'
 import { roleText } from './merchantCenterUtils'
-import { useNavigate } from 'react-router-dom'
+import { MerchantMembersDrawer } from '../components/MerchantMembersDrawer'
 
 type AdminFormValues = {
   username: string
@@ -19,7 +19,9 @@ type AdminFormValues = {
 
 export default function PlatformUsers() {
   const { user: currentUser } = useAuth()
-  const navigate = useNavigate()
+  const [drawerOpen, setDrawerOpen] = useState(false)
+  const [selectedMerchantId, setSelectedMerchantId] = useState<number | null>(null)
+  const [selectedMerchantName, setSelectedMerchantName] = useState<string>('')
   const [tab, setTab] = useState('admins')
   const [admins, setAdmins] = useState<PageResult<User>>({ list: [], total: 0, page: 1, size: 20 })
   const [merchantAccounts, setMerchantAccounts] = useState<PageResult<MerchantMemberGroup>>({ list: [], total: 0, page: 1, size: 20 })
@@ -169,12 +171,21 @@ export default function PlatformUsers() {
     {
       title: '操作',
       width: 160,
-      render: (_, record) => <Button type="link" size="small" onClick={() => {
-        localStorage.setItem('merchant_id', String(record.merchant_id))
-        navigate('/merchant-members')
-      }}>管理商户成员</Button>,
+      render: (_, record) => (
+        <Button
+          type="link"
+          size="small"
+          onClick={() => {
+            setSelectedMerchantId(record.merchant_id)
+            setSelectedMerchantName(record.merchant?.name || record.merchant?.code || `商户 #${record.merchant_id}`)
+            setDrawerOpen(true)
+          }}
+        >
+          管理商户成员
+        </Button>
+      ),
     },
-  ], [navigate])
+  ], [])
 
   const items = useMemo(() => [
     {
@@ -236,6 +247,17 @@ export default function PlatformUsers() {
           {editingAdmin && <Form.Item label="账号状态" name="status" valuePropName="checked"><Switch checkedChildren="启用" unCheckedChildren="停用" /></Form.Item>}
         </Form>
       </Modal>
+
+      <MerchantMembersDrawer
+        open={drawerOpen}
+        merchantId={selectedMerchantId}
+        merchantName={selectedMerchantName}
+        onClose={() => {
+          setDrawerOpen(false)
+          setSelectedMerchantId(null)
+          setSelectedMerchantName('')
+        }}
+      />
     </div>
   )
 }